Popular Wikis

Wikipedia – all-favorite, biggest online encyclopedia
WikiTravel – world-wide travel guide, covers destination guides, hotels and resorts
WikiHow – ‘How-To’ manuals for the problems of everyday life.
WikiBooks – huge collection of user-edited, open-content textbooks and guides.
CookBookWiki – recipes and cooking related wiki. Sections include: dishes, recipes, cuisines and channels
WikiSummaries – short, quick summaries for thousands of books. (Summaries: Freakonomics, Getting Things Done, …, see other bestsellers)
WikiMapia – cool mashup between Google Maps and wiki-style editing. Lets you browse, view, search and add descriptive notes to any location on the globe.
Wiktionary – multilingual, comprehensive, user-edited dictionary. Provides word definitions, etymologies, pronunciations, sample quotations, synonyms, antonyms and translations.
Uncyclopedia – extremely entertaining wikipedia clone, that is filled with funny and not-necessarily correct articles. Check out: Colonel, Britney Spears, Donald Trump,
ProductWiki – collaborative product reviews.
LyricWiki – comprehensive, user-edited source for lyrics. Claims to host lyrics for any song from any artist.
WikiCars – a collaborative guide about cars and all auto-related stuff.
Lostpedia – all about ‘Lost’ serial (episode guides, flashback characters, timeline, etc.)
Dunck – a collaborative guide about Greasemonkey scripts.
Mozillazine – wiki focused on Mozilla’s products (Firefox, Thunderbird, etc) and services.
Operawiki – tips, customization, technical details of the latest version of Opera browser.
Muppet.wikia – for everything related to Jim Henson and the Muppets.
MemoryAlpha – fictional universe of Star Trek
TvIV – encyclopedia of TV shows
MentatWiki – exploring ways to become a better thinker.
Voip-info – a reference guide to all things VOIP, i.e. software, hardware, service providers, reviews, etc.
GPwiki – game programming tutorials and source code for a variety of languages and platforms